Optical fiber sensing technology is often used in large-scale, long-distance monitoring, such as security monitoring used in oil pipelines, high-voltage power grids, pipelines, communications cable and other infrastructure, which the fiber used to be the sensor, real-timely acquiring related disturbance signal, determine the location of the disturbance occurred by the analysis characterize. The structure of single core feedback optical path is: using a single fiber as sensing fiber, the fiber itself is not closed, only apply a feedback device at the end of the fiber, such as a mirror constituting interference optical path. In practice, this structure laying is flexible. The characteristics of such monitoring systems is: light carrying the disturbance information transmitted to the end of the fiber, then reflect by feedback device.
The following is a positioning technology of single core feedback positioning system.
As shown in FIG. 1, we use a sensing section for the optical fiber (optical cable). 1 is the start point of the optical fiber (optical cable), and 2 is a feedback device at the end of the sensing section, such as a mirror. The incident light retrace through the feedback device. Suppose there is a disturbance at point D outside, modulation of light phase is φ(t), when the light twice perturbed points D, phase modulation is subject to:φ1(t)=φ(t)+φ(t−T)
wherein, T=2neffL/c, L is the distance between disturbance point D and feedback device 2, c is the speed of light in vacuum, neff is the effective refractive index of the optical fiber.
As shown in FIG. 2, we configure an interference optical path.
Interference optical path include the following parts: N*M (N, M are integers) coupler 3, P*Q (P, Q are integers) coupler 4, optical fiber delayer 5 (delay τ), an optical fiber (optical cable) 6, and feedback device 2. 3a1, 3a2, . . . , 3aN, 3b1, 3b2 are ports of coupler 3, 3a1, 3a2, . . . , 3aN are co-rotating ports with a total of N, 3b1, 3b2 are two ports in another group co-rotating ports (with a total of M) of coupler 3. 4a1, 4a2, 4b1 are ports of coupler 4, 4a1, 4a2 are two ports in a group co-rotating ports (with a total of P) of coupler 4, 4b1 are two ports in another group co-rotating ports (with a total of Q) of coupler 4. Optical fiber 6 is induction optical fiber. Feedback device 2 make the light transmitted along the fiber go back to the fiber 6 and return to the coupler 4. Light source input through the port 3a1 of coupler 3, after splitting in coupler 3, output respectively through the port 3b1, 3b2, two optical paths is:
: 3b1→5→4a1→4b1→6→2→6→4b1→4a2→3b2
II: 3b2→4a2→4b1→6→2→6→4b1→4a1→5→3b1
The two optical paths join at coupler 3 again and generate interference, interference signals output respectively through port 3a1, 3a2, . . . , 3aN.
In the interference optical path, the light firstly enter delayer 5 and then enter fiber cable 6, the phase modulation applied to the light is:φ2(t)=φ(t−τ)+φ(t−τ−T)
Phase difference between two coherent interference lights is:Δφ=[φ(t)+φ(t−T)]−[φ(t−τ)+φ(t−τ−T)]
In the spectrum of phase difference, there is a frequency drop point, or “notch point”, and we can determine the location of the disturbance arising according to the notch point. “Notch point” is shown in FIG. 3, in this amplitude—frequency diagram obtained by time frequency transform, the “O” mark the notch point. The relationship between notch point and disturbance position is:
                    f        null            ⁡              (        k        )              =                  k        2            ·              c                  2          ⁢                      n            eff                    ⁢          L                      ,      (                  k        =                              2            ⁢            n                    -          1                    ,              n        ∈        N            
wherein, fnull(k) is frequency of k-order notch point.
We can see from the above principle, the coherent light must transmit from the endpoint 1 of sensing optical fiber 6 to endpoint 2 and then return to sensing optical fiber 6, in order to carry the position “L” message. However, in practice, due to the structural characteristics of the optical fiber and the fiber itself defects and other reasons, there is a scattered light in optical fibers, such as Rayleigh scattering light and the like.
As shown in FIG. 4, 7 is a scatter point, backscattered light along the optical cable go back to interference structure, and therefore there is two beams:
I: 3b1→5→4a1→4b1→6→7→6→4b1→4a2→3b2
II: 3b2→4a2→4b1→6→7→6→4b1→4a1→5→3b1
Because of similar spectral characteristics, the optical path are equal without disturbance, and therefore join at the coupler 3 again will also occur interference. Obviously, the information carried by the two beam of interference light is the length L7 between point 7 and disturbance point D. 8 is another scattering point, the length information carried by the interference formed by backscatter is the length L8 between point 8 and disturbance point D, apparently, L7≠L8≠L, since these interference is mixed at the output, the interference light generated by Brillouin backscattered light or Raman backscattered light can be filter out by optical filter, but for the interference light generated by Rayleigh scattering light, or the interference light generated by contact point of optical path, it is impossible to eliminate by optical filtering method, will affect the purity of useful interference signal, and will directly affect the accuracy of the disturbance L position. Generally, the intensity of interference generated by backscattered light, contact reflected is significantly less than the intensity of interference generated by reflected light (effective interference signal), and will not have a significant impact on the effective interference signal, accuracy of L can meet the actual needs. But after the monitoring circuit reach a certain length, scattered light affects the entire line obviously, then we can observe the obvious interference signal distortion has occurred, the system can not obtain a valid interference signal normally.
Similarly, reflection by the contact point of optical path can also cause the same adverse effects on the interference signal.
The impact of scatter (reflect) light in the conventional path is not only the obvious restriction in monitoring system. When a large scatter (reflect) point exists, the system can not be properly tested in the line.
In order to cut the impact of the signal, the invention 201010508357.2 (FIG. 5) is proposed by use phase generated carrier technology to separate effective interference phase information from optical output mixed with backscattered light, contact point reflected light interference signal to obtain pure signal having effective disturbance position information, so as to eliminate the impact of back scattered light and the like purposes. In the technology, at the end of sensing optical fiber (optical cable) 6, access a phase modulator 9 close to the feedback device 2, apply modulation signal to phase modulator 9 to obtain carrier fundamental frequency (or double frequency) sideband signal only contains useful information is extracted, and the use of PGC demodulation side information extracted. PGC demodulation techniques generally use coherent demodulation technique in which demodulation process requires the use of homologous signal modulation signal as a reference signal. Due to the need to obtain a modulated signal at signal generation side, in the application of single feedback system, when the end with a signal modulation (position of 9) away from the signal generation side, how to obtain a reference signal, the method becomes difficulty to achieve.